The present invention relates to a method and apparatus for operating a fuel cell that improves the overall efficiency of the fuel cell system. In particular, efficiency is improved by controlling the supply of oxidant so as to reduce excess oxidant flow.
Electrochemical fuel cells convert reactants, namely, fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions.
The fuel fluid stream, which is supplied to the anode, typically comprises hydrogen and may be pure gaseous hydrogen or a dilute hydrogen stream such as a reformate stream. Alternatively, other fuels such as methanol or dimethyl ether may be supplied to the anode where such fuels may be directly oxidized. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, and may be pure gaseous oxygen, or a dilute oxygen stream such as air.
For a fuel cell, reactant stoichiometry is defined herein as the ratio of the reactant supplied over the reactant theoretically required to produce the current produced by the fuel cell. For conventionally operated fuel cells which typically supply a surplus of oxidant to the cathode, since the oxidant is preferentially reduced at the cathode, oxidant stoichiometry is commonly expressed as the ratio of the oxidant supplied over the oxidant consumed. However, at lower stoichiometries, the reduction of oxidant may not be responsible for all of the current produced by the fuel cell. Other reactions, such as, for example, the reduction of protons may also occur at the cathode and contribute to the current output (i.e. with the consequence of reduced output voltage). In this example, while oxidant may still be the main component being reduced at the cathode, the amount of oxidant theoretically required to produce the current output may be higher than the amount of oxidant actually supplied. Therefore, when components other than the oxidant are reduced at the cathode, oxidant stoichiometries less than one may be sustained. If the oxidant is a dilute oxidant stream such as air, only the reactant component, namely oxygen, is considered in the calculation of stoichiometry (that is, oxidant stoichiometry is the ratio of the amount of oxygen supplied over the theoretical amount of oxygen required to produce the fuel cell output current).
Hydrogen and oxygen are reactive in the fuel cell and are particularly reactive with each other. Accordingly, in solid polymer fuel cells, an important function of the membrane electrolyte is to keep the hydrogen supplied to the anode separated from the oxygen supplied to the cathode. In addition, the membrane is proton conductive and functions as an electrolyte.
The overall efficiency of a fuel cell system is a function of the total power output of the fuel cell(s) and the parasitic power consumption. Total parasitic power consumption is defined herein as the sum of all power that is consumed by the fuel cell system in the course of generating electrical power. The net electrical power output is the total power output minus the total parasitic power consumption. Therefore, overall efficiency may be improved by reducing the parasitic power consumption.
One source of parasitic power consumption, for example, is the oxidant delivery subsystem that typically employs a mechanical device such as a compressor, fan, pump, rotary piston blower, or an equivalent mechanical device that consumes power to supply oxidant to the fuel cell. Higher oxidant stoichiometries generally result in higher parasitic power consumption because more power is generally required to deliver more oxidant to the cathode. Conventional fuel cell systems typically operate with an oxidant stoichiometry greater than two (2.0). Since conventional fuel cell systems direct at least twice the amount of oxygen to the cathode than is actually required to satisfy the electrical power demand, a significant amount of the parasitic power consumption is for directing surplus oxygen to the cathode. Further, fuel cell systems commonly employ a dilute oxidant stream. A dilute oxidant stream is defined herein as a fluid stream that comprises less than 100% oxidant. For example, air is a dilute oxidant stream that typically comprises about 20% oxygen, in addition to other components such as nitrogen. Accordingly, when air is employed as the dilute oxidant stream, parasitic power consumption is amplified because in addition to the surplus oxygen, the oxidant delivery subsystem must also supply a proportionate amount of the other non-reactive components.
One reason why the parasitic power consumption associated with high oxidant stoichiometries is tolerated, is that excess oxidant is desired at the cathode to avoid oxidant starvation at the cathode electrocatalyst. Oxidant starvation is defined herein as the status when oxidant stoichiometry is less than one. Oxygen starvation typically results in a condition, in the absence of oxidant at the cathode electrocatalyst, favoring the production of molecular hydrogen from protons and electrons at the cathode. In severe cases of oxidant starvation the fuel cell may generate a negative voltage and this condition is known as cell reversal. Oxidant stream delivery systems are typically designed to provide a generous surplus of oxidant to maintain performance, to reduce the likelihood of oxidant starvation, and to reduce the likelihood of hydrogen production at the cathode, even though this results in the aforementioned amplified parasitic power consumption.
In fuel cells, oxidant starvation is most likely to occur in regions furthest downstream from the cathode inlet where the oxidant stream enters the cell, for example, near the cathode outlet. An oxidant stoichiometry that provides a surplus of oxidant to the fuel cell provides an adequate concentration of oxidant to the electrocatalyst throughout the electrochemically active area of the cathode, including near the cathode outlet.
Another reason why conventional fuel cell systems seek to avoid low oxidant stoichiometries is that the temperature within the fuel cell may rapidly increase when oxidant stoichiometry is too low. It is generally desirable to maintain the temperature of solid polymer fuel cells below 100xc2x0 C. When the temperature increases within the fuel cell, parasitic power consumption increases because of the higher load on the cooling system, offsetting, to some degree, the reduction in parasitic power consumption associated with operating at a lower oxidant stoichiometry.
Another disadvantage of operating a fuel cell system with a high oxidant stoichiometry is that higher oxidant stoichiometries generally require higher speeds for the mechanical devices used by the oxidant delivery subsystem to supply the oxidant stream to the cathode. Now that fuel cell systems are being developed for commercial use, mechanical considerations over the planned lifetime of commercial fuel cell systems are a factor. A mechanical disadvantage of conventional high stoichiometry methods of operation is that such methods may result in increased wear and more frequent maintenance. If the oxidant is air, there may be additional operational costs because supplying a high surplus of oxidant also results in higher flow rates that may increase air filter maintenance and/or reduce filter efficiency.
A method of operating a fuel cell system controls oxidant stoichiometry to reduce parasitic power consumption to improve overall efficiency, while avoiding low oxidant stoichiometries that might cause reduced performance, cell reversal, hydrogen production at the cathode, and increased heat generation within the fuel cell. The fuel cell system comprises a fuel cell power generating subsystem having at least one fuel cell, and an oxidant delivery subsystem that comprises at least one mechanical device for supplying an oxidant stream to a cathode of the fuel cell. The fuel cell also has an anode supplied with a fuel stream. In a preferred embodiment, the fuel cell is a solid polymer fuel cell.
The method comprises controlling the mechanical device, to reduce parasitic power consumption by reducing the oxidant stoichiometry until dV/d(OS) is greater than a predetermined value (xe2x80x9cPVxe2x80x9d), where dV is the change in cell voltage and d(OS) is the change in oxidant stoichiometry (that is, the slope of a plot of voltage as a function of oxidant stoichiometry). Cell voltage is measured in volts and oxidant stoichiometry is a unit-less ratio.
To practice the invention, the value of dV/d(OS) need not actually be calculated if a relationship between dV/d(OS) and another operational characteristic is known. For example, in preferred embodiments, an operational characteristic that correlates to dV/d(OS) and/or oxidant stoichiometry may be monitored. The fuel cell system is controlled to take action when the value of the monitored operational characteristic correlates to when dV/d(OS) is equal to or greater than PV. For example, in a typical fuel cell system, during normal operation, current density is kept constant and when oxidant stoichiometry is being reduced, a particular cell voltage correlates to when dV/d(OS) increases to PV. That is, when cell voltage decreases below a threshold voltage, this is determined when dV/d(OS) is higher than PV. Accordingly, a fuel cell system may be operated to reduce parasitic power consumption by controlling the oxidant delivery subsystem to maintain voltage output within a predetermined voltage range which typically corresponds to an oxidant stoichiometry range between about one and two, wherein PV is selected so that dV/d(OS) equals PV at the lower limit of the selected voltage range. The preferred range for oxidant stoichiometry may actually change according to the instantaneous operating conditions. For example, when a fuel cell is operating in an idle or low output condition, a higher oxidant stoichiometry may be preferred to prevent accumulation of water at the cathode. Accordingly, the value of PV may be dynamic.
The characteristics of the fuel cell and/or the type of reactants may also influence the preferred oxidant stoichiometry range. For example, in a direct methanol fuel cell, higher stoichiometries are typically employed, but the reactant supply may still be controlled to prevent dV/d(OS) from increasing to higher than PV (although for a direct methanol fuel cell, PV will correspond to a higher stoichiometry, compared to a fuel cell which is fed hydrogen gas or reformate as the fuel stream).
Similarly, when current density is constant and oxidant stoichiometry is being reduced, a particular oxidant stoichiometry correlates to when dv/d(OS) increases to PV. Accordingly, operational characteristics, such as the oxygen concentration in the cathode exhaust stream, which correlate to oxidant stoichiometry, may be monitored to determine when oxidant stoichiometry is reduced to a value which correlates to when dV/d(OS) increases to greater than or equal to PV. The oxidant concentration in the oxidant supply stream is typically known, but if the oxidant supply stream has a variable oxidant concentration (for example, if an oxidant enrichment system is employed), the method may further comprise monitoring and measuring the oxidant concentration in the oxidant supply stream, in addition to monitoring and measuring the oxidant concentration in the oxidant exhaust stream. Alternatively, oxidant stoichiometry may be determined by monitoring and measuring a different operational characteristic, such as, for example, current output for the fuel cell power generating subsystem, which, in addition to the oxidant concentration in the oxidant exhaust stream may be used to calculate oxidant stoichiometry.
The value of dV/d(OS) generally increases as oxidant stoichiometry and cell voltage both decrease. In one embodiment, PV corresponds to when oxidant starvation is beginning to occur or when oxidant starvation is beginning to cause a decline in performance. In a more preferable embodiment, PV corresponds to when further reductions in oxidant stoichiometry will cause a sharp decline in cell voltage output, for example, when dV/d(OS) is higher than 0.02 volt. Preferably, PV is between 0.3 volt and 7.0 volts, so that the fuel cell system operates mostly when dV/d(OS) is less than PV. The selected value for PV controls the oxidant stoichiometry so that it is kept between about one and two during normal operation and closer to about one or a predetermined target value, preferably between 1 and 1.5, during steady state operation.
In a preferred apparatus for practising the method, the fuel cell is one of a plurality of fuel cells arranged in a stack. When the method is applied to a fuel cell stack, the sensor may monitor the operational characteristic for one or more individual fuel cells and/or for the stack as a whole. The sensor may thus be located to monitor an operational characteristic (for example, oxidant or hydrogen concentration) within a portion of a reactant passage (for example, an internal cathode exhaust passage) that is disposed between the outside end surfaces of the stack end plates.
The oxidant stoichiometry is preferably controlled by controlling the oxidant stream mass flow rate, for example, by controlling the speed of a mechanical device, such as a compressor, a fan, a pump, or a blower. Reducing the speed of the mechanical device generally reduces parasitic power consumption and reduces oxidant stoichiometry. However, alternate methods of controlling oxidant stoichiometry may be employed which also reduce parasitic power consumption. For example, if an oxidant enrichment subsystem is employed, oxidant stoichiometry may be controlled by increasing or decreasing the concentration of oxidant in the oxidant stream supplied to the cathode(s) of the fuel cell power generating subsystem. Another method of controlling oxidant stoichiometry is adjusting the electrical power output of the fuel cell, wherein reducing power output generally increases oxidant stoichiometry.
A preferred method that employs a hydrogen sensor (the xe2x80x9cHydrogen Sensor Methodxe2x80x9d) comprises:
(a) monitoring a cathode exhaust stream downstream of the cathode to detect hydrogen gas concentration; and
(b) decreasing oxidant stoichiometry when the hydrogen gas concentration is less than a first threshold concentration.
The Hydrogen Sensor Method may further comprise increasing the oxidant stoichiometry when the hydrogen concentration is higher than a second threshold concentration (for example, 20 ppm of hydrogen), which correlates to operating conditions which are indicative of actual or potential oxidant starvation. The first threshold concentration may be, for example, the lower detection limit of the hydrogen sensor that is used to monitor the cathode exhaust stream. The second threshold concentration is greater than the first threshold concentration. When the hydrogen concentration is between the first and second threshold concentrations, the controller does not take any action to adjust the oxidant stoichiometry.
A problem with using the hydrogen concentration measured in the cathode exhaust stream to detect oxidant starvation is that oxidant starvation is not the only possible cause for hydrogen gas being detected at the cathode. For example, when the fuel comprises hydrogen, holes or cracks may form in the membrane or seals and permit reactants to xe2x80x9ccross overxe2x80x9d from the anode side to the cathode side, and vice versa. If significant reactant crossover is detected, the conventional response is to shut down the fuel cell so that it may be repaired or replaced. Fuel crossover and oxidant starvation may both cause reduced fuel cell performance, but the detection of one condition requires a response which is different from the response required for the other condition. Oxidant starvation causing hydrogen to be produced at the cathode generally requires the oxidant stoichiometry to be increased, whereas fuel crossover, if significant, may require the fuel cell to be shut down. Therefore, for appropriate action to be taken, it is desirable for the controller to be able to distinguish between oxidant starvation and fuel crossover when the fuel comprises hydrogen. The following embodiments of the Hydrogen Sensor Method provide procedures for distinguishing between oxidant starvation and fuel crossover.
The Hydrogen Sensor Method may further comprise steps for reducing the hydrogen gas concentration within the cathode exhaust stream when the hydrogen gas concentration is greater than a second threshold concentration, wherein the steps comprise comparing the oxidant stream mass flow rate to a maximum desired mass flow rate, and
(a) if the oxidant stream mass flow rate is less than the maximum desired mass flow rate, increasing the oxidant mass flow rate (that is, if raising the oxidant mass flow rate results in less hydrogen being detected at the cathode, then it is confirmed that oxidant starvation was likely the reason for hydrogen being detected; if oxidant starvation is not the source of the hydrogen at the cathode, the oxidant stream mass flow rate will quickly increase to the maximum desired mass flow rate and the controller will determine that fuel crossover is the likely hydrogen source); and
(b) if the oxidant mass flow rate is already greater than or equal to the maximum desired mass flow rate,
ceasing operation of the fuel cell if the hydrogen gas concentration is greater than a third concentration threshold which is greater than the first and second concentration thresholds (that is, since the oxidant stream mass flow rate is already at, or exceeds, the desired maximum, oxidant starvation is not the likely source of the hydrogen in the cathode exhaust stream; since the hydrogen concentration is above the third threshold, this indicates that there may be an excessive amount fuel passing through leaks between the anode and cathode); and
generating a warning signal and continuing to operate the fuel cell if the hydrogen gas concentration is less than the third concentration threshold (that is, the value of the third threshold is selected so that the fuel cell system can be safely operated when the hydrogen concentration in the cathode exhaust stream is less than the third threshold).
The Hydrogen Sensor Method may further comprise continuously monitoring the cathode exhaust stream for the hydrogen gas concentration and determining whether the hydrogen gas concentration is increasing or decreasing, and when the hydrogen gas concentration is greater than a second threshold concentration, the method further comprises:
maintaining a substantially constant oxidant stoichiometry when the hydrogen concentration is decreasing; and
increasing the oxidant stoichiometry when the hydrogen concentration is increasing.
In addition to monitoring whether hydrogen concentration is increasing or decreasing, when the hydrogen gas concentration is greater than a second threshold concentration, the method may also comprise additional steps to determine whether the source of the hydrogen is oxidant starvation or fuel crossover. For example, the additional steps may comprise:
measuring fuel cell voltage and comparing it to a voltage threshold value (for a Ballard(copyright) MK V fuel cell, the voltage threshold value could be, for example, 100 millivolts), and
if the fuel cell voltage exceeds the voltage threshold value and the hydrogen gas concentration is increasing, decreasing the pressure of the fuel stream (in this case, since voltage exceeds the threshold value, the reason for the increasing hydrogen concentration is probably a leak; to reduce the effect of the leak, the method preferably comprises controlling the fuel stream pressure so that it is less than or equal to the pressure of the oxidant stream);
if the fuel cell voltage is less than the voltage threshold value, the hydrogen gas concentration is increasing, and oxidant mass flow rate is less than a desired maximum, then increasing the oxidant stoichiometry (in this case, since oxidant mass flow rate is less than the desired maximum, the cause for the low cell voltage and the presence of hydrogen gas may be oxidant starvation, and the controller attempts to correct this condition by increasing the oxidant stoichiometry); and
if the fuel cell voltage is less than the voltage threshold value, the hydrogen gas concentration is increasing, and oxidant mass flow rate is greater than or equal to a desired maximum, then decreasing the pressure of the fuel stream (in this case, the low cell voltage may be caused by oxidant starvation or fuel leaking from the anode to the cathode; since the oxidant mass flow rate is already greater than or equal to the desired maximum, the pressure of the fuel stream is reduced, thereby reducing fuel cell power output and oxygen consumption at the cathode, to counter oxidant starvation and reduce the effect of any leaks).
Further additional steps may be taken to confirm whether the detected hydrogen gas concentration is caused by oxidant starvation or fuel crossover. For example, the method may also comprise regulating fluid pressure of the oxidant and fuel streams to increase or decrease a pressure differential between the oxidant and fuel streams to help determine whether the hydrogen measured at the cathode is caused by a leak or by oxidant starvation. If the change in the pressure differential has a significant effect on the measured hydrogen concentration, then it can be determined that there is a significant problem with hydrogen crossover.
For any of the above-described methods, the oxidant stoichiometry is typically adjusted by controlling the speed of the oxidant compressor or blower. However, other methods of changing the oxidant stoichiometry may also be used, such as adjusting the oxidant concentration in the oxidant supply stream or changing the electrical power output of the fuel cell without changing the mass flow rate of the oxidant supply stream. When the oxidant stream mass flow rate is adjusted, it is typically changed by a fixed amount or by a fixed percentage of the instant oxidant stream mass flow rate. Alternatively, oxidant stoichiometry may be adjusted by adjusting the oxidant stream mass flow rate by an amount that is dependent upon the magnitude of the detected hydrogen gas concentration. For example, the controller may be programmed to reduce the oxidant stoichiometry by a larger amount when a large surplus of oxygen is detected compared to when only a small surplus of oxygen is detected.
The method of controlling the oxidant delivery subsystem to reduce parasitic power consumption may comprise calibrating an oxidant delivery subsystem for a fuel cell. For example, the calibration method may comprise:
(a) operating the fuel cell at a particular electrical power output;
(b) supplying an oxidant stream to a cathode of the fuel cell;
(c) adjusting the operating speed of a mechanical oxidant delivery device;
(d) measuring an operational characteristic that corresponds to dV/d(oxidant stoichiometry); and
(e) recording as the desired operating speed for said particular electrical power output, said operating speed when said dV/d(oxidant stoichiometry) is equal to a predetermined value.
The calibration method may be repeated for a plurality of electrical power outputs so that the desired operating speed for the mechanical oxidant delivery device may be determined and recorded in a look-up table for many different electrical power demands. The desired operating speed may then be determined by referring to a look-up table for the operating speed that corresponds to the instant electrical power demand.
An advantage of the calibration method is that it may be used throughout the operating life of the fuel cell to adjust for changes in the fuel cell over time. For example some of the fuel cell properties may be subject to degradation over time and that may change the stoichiometry requirements over the operational lifetime of the fuel cell.
The present method and apparatus also controls the amount of oxidant supplied to a fuel cell stack and reduces system inefficiencies caused by the over-supply of oxidant. Preferably, the method also controls the oxidant delivery subsystem to increase the oxidant stoichiometry to avert oxidant starvation conditions. Accordingly, a method of operating a fuel cell is provided that detects when the oxidant stoichiometry may be decreased or increased, and when the flow of oxidant should be discontinued altogether.
The present method may also be employed to operate a fuel cell and further reduce parasitic power consumption by controlling the supply of fuel to reduce excess fuel flow. The same principles that apply to the present method for controlling the oxidant supply apply to a method for controlling the fuel supply. Fuel cells typically employ a mechanical device, such as, for example, a compressor or pump, to supply a fuel stream to the anode(s) of the fuel cell(s). Therefore, parasitic power consumption may be reduced by reducing fuel stoichiometry to reduce the amount of excess fuel supplied to the fuel cell anode(s) and the work performed by the compressor. A reduction of the fuel stoichiometry generally causes an increase in dV/d(fuel stoichiometry). According to the present method, fuel stoichiometry is kept within a predetermined range by reducing fuel stoichiometry until dV/d(fuel stoichiometry) increases above a predetermined threshold value. The predetermined range and threshold value depend upon the particular characteristics and operating conditions of each particular fuel cell or fuel cell stack. The predetermined range may be empirically determined, for example, with consideration to these factors.
Generally, it is desirable to reduce reactant stoichiometry until dV/d(reactant stoichiometry) is greater than about 0.02 volt. More preferably, the predetermined value for dV/d(reactant stoichiometry) is between 0.30 and 7.0 volts. The voltage drop is generally more severe with fuel starvation, compared to oxidant starvation and when cell voltage is one of the monitored characteristics, this effect may be used to help differentiate between oxidant or fuel starvation.